1. Field of the Invention
The present invention relates to a tracking servo circuit in an optical disc player and, more particularly, to a tracking servo circuit suitably used for correcting an offset of a tracking error signal caused by a sensitivity imbalance in a tracking servo photo diode when a so-called three-spot method is used for generating the tracking error signal.
2. Description of the Related Art
In an optical disc player, correctly reading pit information formed on a disc requires tracking servo control for controlling a relative position of a light beam spot across the disc so that a track along which information pits are arranged is correctly traced.
In the above-mentioned tracking servo control, based on a beam reflected from a disc surface on which the information track is formed, a deviation of the light beam spot relative to the track across the disc is detected as a tracking error signal. Then, a tracking actuator is driven by a voltage corresponding to an amplitude of the tracking error signal to continuously correct the beam spot position so that the spot is always controlled to trace the track at its center line.
Known as a typical way of producing the tracking error signal is the three-spot method.
In this method, three light beams are projected onto the surface of the disc to form three spots along a same straight line at a same interval. The straight line is always kept at a certain angle to the disc track.
Of the three light beams, a central beam or a main beam is used for information reading and focus servo control and two side light beams are used for tracking servo control.
The two side light beams are reflected from the disc surface to be received by two photodiodes. A difference between outputs of the two photodiodes is used as the tracking error signal.
Ideally, the tracking error signal is zero when the main beam spot is positioned at a center of the track and, as the spot departs from the center, an amplitude of the signal increases in a polarity in which the departure takes place, thereby presenting a characteristic of a generally "S" curve.
However, if there is a difference in sensitivity between the two tracking servo control photodiodes, a direct current offset is caused in the tracking error signal, preventing the signal from getting zero although the main beam spot is located at the center of the track.
Therefore, some countermeasures need be taken to prevent this problem from affecting a disc player reliability.
Techniques for cancelling the direct current offset in the tracking error signal are disclosed in Japanese Non-examined Patent Publication Nos. 60-61925 and 4-34209 for example.
In the technique of No. 60-61925, a desired track is searched for by scanning the light beam spot across tracks. An offset voltage is detected from a tracking error signal obtained by the scanning. And the offset voltage is used to correct the tracking error signal.
In the technique of No. 4-34209, a track-to-track jump is performed over particular tracks to detect an offset voltage from a track error signal generated by the track jump. Each time an offset voltage is detected, a previously held offset voltage is updated. The tracking error signal is corrected by the updated offset voltage.
In the former technique, however, detection of the offset voltage from the tracking error signal generated when the light beam stop is scanned across a track requires to set a particular mode for making the beam spot cross the track.
In the latter technique, since the offset voltage is detected from the tracking error signal generated when a track-to-track jump is performed, the offset voltage cannot be detected if no jump is performed and therefore the track error signal cannot be corrected.